The present invention relates to on-press developable negative-working printing plate precursors, which can be exposed by UV, visible, and infrared radiation. In particular, the present invention relates to on-press developable printing plates precursors having a radiation sensitive layer including an initiator system and polymeric binders containing polyethylene oxide (“PEO”) segments.
Lithographic printing plate precursors typically comprise a radiation-sensitive coating applied over the hydrophilic surface of a support. Radiation-sensitive coatings generally include photosensitive components dispersed within an organic polymeric binder. After a portion of the coating is exposed to radiation (commonly referred to as imagewise exposure), the exposed portion becomes either more developable or less developable in a particular liquid than an unexposed portion of the coating. A printing plate precursor is generally considered a negative-working precursor if the exposed portions or areas become less developable in the developer and the unexposed portions or areas are removed in the developing process. After being developed in a suitable liquid, the image area accepts ink, while the revealed regions of the substrate's hydrophilic surface repel ink.
There are several potential ways of improving the properties of radiation sensitive compositions to enhance printing plate performance. One method of improvement involves optimizing the radiation sensitive components in the radiation sensitive layer. For example, a variety of references report the use of initiator systems or complexes that include various combinations of free-radical initiating compounds and radiation absorbing materials. Upon exposure of the radiation sensitive layer to radiation, the radiation absorbing compound absorbs the radiation and releases heat energy. The free-radical initiating compound promotes polymerization or hardening of a polymerizable material to produce an image area.
For example, U.S. Published Application No. 2002/0025489 to Shimada et al. reports a heat-sensitive composition including a compound that generates an acid or radical when heated (e.g. an onium salt) and a compound whose physical properties are irreversibly changed by an acid or radical. The composition may further include an IR dye. U.S. Published Application No. 2003/0054288 to Shimada et al. reports a heat sensitive composition including a cationic onium salt, a compound having a polymerizable unsaturated group and a light-heat converting agent such as an IR dye. U.S. Published Application No. 2003/0068575 reports a photosensitive layer for a printing plate including an IR absorbing agent, an onium salt, a radically polymerizable compound, a polymeric binder and an organic dye.
Additionally, U.S. Pat. No. 4,751,102 to Adair et al. and U.S. Pat. No. 4,937,159 to Gottschalk et al. report photohardenable compositions including a free-radical polymerizable or crosslinkable compound and an ionic dye-counter ion compound capable of absorbing radiation and producing free radicals to initiate polymerization or cross-linking of the polymerizable or crosslinkable compound. U.S. Pat. No. 5,368,990 to Kawabata et al. reports a photopolymerizable composition comprising an addition polymerization compound and a photopolymerization initiating compound including a specific anionic dye and a diaryliodonium salt as a polymerization initiator. U.S. Pat. No. 5,208,135 to Patel et al. reports an anionic photosensitive dye, an iodonium salt and a free-radical curable resin.
More recently, it has been determined that initiator systems or complexes may be utilized in “processless” or “on-press developable” printing plates. As used herein, the terms “processless” and/or “on-press developable” refers to printing plate precursors that do not require one or more conventional processing steps (e.g. development) prior to mounting on a printing press. For example, U.S. Pat. Nos. 6,482,571 and 6,548,222 to Teng report on-press developable printing plates having a thermosensitive layer including a free radical initiator, a radiation absorbing material and a polymerizable monomer.
Despite the recent advances in processless printing plates, it would be beneficial to prepare a processless printing plate precursor incorporating an initiator system that promotes improved manufacturing efficiency, including faster imaging speeds and improved handling and evaluation characteristics (e.g visible printout), as well as substantially increased durability and run lengths on-press.